The present invention relates to a soft-focus lens system for use in photography.
Various types of soft-focus lens systems have been proposed and they are roughly divided into two types. The first of these types is a system having spherical aberration intentionally left in it which has not been compensated for. The other type is a system having chromatic abberation left in it which has not been compensated for. The second type is generally referred to as the "chromatic abberation type" and is unsuitable for color photography and is not commonly employed in modern cameras. Therefore, soft-focus lens systems used today are generally of the first type. This "spherical aberration type" is subdivided into: (A) a tertiary aberration type (e.g., IMAGON of Rodenstock Instruments Corporation, 200 mm, H5.8; and SMC PENTAX SOFT of Asahi Kokaku Kogyo Kabushiki Kaisha, 85 mm, F 2.2); (B) a higher-order aberration type, e.g., the systems described in JP-A-52-141223 and JP-A-56-50309 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"); and (C) an adjustable higher-order aberration type, e.g., the systems described in JP-A-54-156531 and JP-A-55 -129308.
The construction of type (A) is fairly simple (it is mostly constructed of a one-group-two-element composition) and the soft-focusing effect is normally controlled by adjustment of a diaphragm stop. However, in most systems of type (A), the compensation for curvature of the field and astigmatism are frequently insufficient to ensure a wide viewing angle. If one strains to widen the viewing angle, coma will occur so that the sharpness of the image is impaired.
Type (B) has the advantage that focusing is easily accomplished compared to focusing a type (A) system. However, in order to produce higher-order aberrations, lens surfaces of strong curvature are necessary. This causes an increase in lens thickness and sensitivity to unwanted phenomena such as eccentricity. As a result, an increase occurs not only in the size of the lens system but also in the manufacturing cost.
The main feature of type (C) is that it is capable of adjusting the soft-focusing effect with the diaphragm stopped down to the same value. However, this type requires a complex lens composition and lens moving mechanism and its manufacturing cost is considerably higher than that or types (A) and (B).